Guess the failure?

[GraemeSmith]

A game in which I offer you a real world issue, you can discuss it (if you want) and I'll reveal the answer or confirm who got it right after a few posts.

Tight well running AeroVee. 3 hours out of condition inspection with newly serviced valves.
Aileron Roll left - Good. Aileron Roll right - Good. Loop - and as I pulled out the bottom of the loop the engine started sounding rough.
Straight and level and adjusted mixture to leaner (sometimes my loops finish a bit rich till I get it dialed in). No improvement.
It SOUNDS like I lost a cylinder.
The engine still seems to be reasonably smooth and producing good power.
EGT1 has dropped off the scale and has no EGT.
CHTs are all good and normal
No undue vibration
In flight Mag check - normal.
Sounds terrible.
Precautionary landing back at base and decowl.

What did I find?

[DCASonex]

Exhaust pipe for one cylinder failed or fell off.

[Bryan Cotton]

I'd guess a weld failure at the #1 exhaust flange.

[Dave Wolfe]

Ill go with: loose intake at the head

[Murray Parr]

#1 EGT has blown out of the pipe

[onex28]

EGT wire disconnected.

[GraemeSmith]

Most of you are in the ball park.

Engine running and power there and mags OK - It is probably not anything till you get to the exhaust port.

Then a broken flange / dropped off pipe / missing EGT probe would all cause a drop in EGT and a change in sound as the exhaust was no longer going down the pipe.

In fact it was a missing (as in used to be there but no longer.....) exhaust gasket. It was brand spanking new 3 hours prior and looked fine going on - but was no longer there anymore. Metal/fibre/metal sandwich type - just some eroded shreds of metal left around the exhaust bolts. That might speak to me not torquing the up properly - I don't THINK that is the case - but ?????

New gasket and 2.5 hours and no repetition or sign of repetition.

[Bryan Cotton]

That answer was a surprise. I was thinking Murray had it right.

[pappas]

Graeme,

That was interesting. It had very similar symptoms to the engine failure that put me in the desert in 2021. Cylinder #4 EGT went immediately stone-cold. However, I could not maintain altitude as the engine was not producing enough power for that.

EGT1 has dropped off the scale and has no EGT. - Check
CHTs are all good and normal. - Check
No undue vibration - Check
In-flight Mag check - normal. - Check
Sounds terrible. - Check

But, mine was the broken #4 Exhaust valve adjuster foot. My exhaust valve closed shut, would not exhaust the combustion remnants, and started backing up exhaust gasses and fresh fuel-air mix into the intake manifold upstream into #3. Just one big road block. I am glad your outcome was so much better than mine! Fun Game!

[Dave Wolfe]

My first thought was Murray's idea. Then I thought nope too obvious.

The only thing similar that I experienced was having a spark plug unscrew out of a lycoming 6 cylinder. It was loud and shook but still made plenty of power. That also caused a bit of oil to come onto the windshield.

The only engine failure was a newer model cessna 172 with the 160 hp injected io-360 I think. Engine stopped during a power off stall because the idle mixture went way too rich. Got it restarted by not following the procedure but rather cranking with the mixture all the way back. There was a service advisory shortly after that.